Device and method for alignment of vertically stacked wafers and die

ABSTRACT

A device is provided that includes a first die having a first alignment structure that includes a plurality of first transmission columns arranged in a pattern and a second die positioned on the first die, the second die having a second alignment structure that includes a plurality of second transmission columns arranged in the same pattern as the first transmission columns. The first and second transmission columns are each coplanar with a first surface and a second surface of the first and second die, respectively.

BACKGROUND

1. Technical Field

The present disclosure relates to alignment structures formed on aplurality of wafers configured to be aligned to form a multichip stackin a single package.

2. Description of the Related Art

As consumer demand increases for smaller multifunction devices,manufacturers face significant challenges to integrate differentsemiconductor technologies on a single die. Multichip packages havebecome increasingly popular to increase device density and to combinetraditionally incompatible technologies, such as logic, memory, andmicro-electromechanical systems (MEMS). For example, as cell phonesmorph into personal entertainment systems, manufacturers look for waysto integrate multiple technologies, like SRAM, DRAM, flash, logic,analog, and radio frequency, into one relatively thin package.

Multichip packages also address some of the limitations that have arisenwith respect to two-dimensional scaling. The multichip packages may bealigned and bonded at the wafer level or as individual die. Each of thedie to be included in a multichip package may be formed on a singlewafer dedicated to a particular technology. For example, one die may bemanufactured to be a processor that is configured to be packaged with aMEMS sensor, which is separately manufactured on another wafer.

These vertically stacked chips formed from multiple die offer improveddensity and performance. The challenges to integrate traditionallyincompatible processes on a single wafer are avoided by formingincompatible technologies on individual wafers and packaging them in thesingle package.

FIG. 1 is a vertical stack 10 of six wafers 12 a-12 f aligned and bondedaccording to a known method of alignment. Each wafer 12 a-12 f is formedindependently from the other wafers to have a plurality of throughsilicon vias (TSVs) 14 in similar locations. After the processing toform a plurality of die on the wafers 12 a-12 f is complete, the wafers12 a-12 f are thinned to approximately 70 microns. The thinning reducesthe size and weight of the final vertically stacked multi-chip package.

The TSVs 14 can be used for alignment and to form electricalcommunications between wafers. The TSVs 14 may be formed by deepreactive ion etching before or after the wafers 12 a-12 f are thinned.Typically, the TSVs are formed after the devices are formed on thewafers. The TSVs 14 in FIG. 1 are annular metal TSVs that are shown incross-section as two vertical pillars 16 through each wafer 12 a-12 f. Atop pad 18 and a bottom pad 20 are formed on a top and bottom surface ofeach wafer 12 a-12 f after the wafer is thinned. The top and bottom pads18 and 20 are solder interconnects that electrically connect the TSVs 14of the wafers 12 a-12 f.

The wafers 12 a-12 f in FIG. 1 are not accurately aligned. For example,the annular TSVs 14 are all formed to have the same diameter. In thecross-section of FIG. 1, a width between each pillar 16 of each TSV 14varies from wafer to wafer. More particularly, the width between thepillars 16 of the TSVs 14 of wafer 12 a is smaller than the widthbetween the pillars 16 of wafer 12 b. This variation in width indicatesthat the TSVs 14 of each wafer 12 a-12 f are not aligned.

Alignment is achieved when the top pads 18 are in contact withrespective bottom pads 20 of an adjacent wafer. The inaccuracy of thismethod of alignment is also shown by considering a central axis 22 ofeach annular TSV 14. If the wafers 12 a-12 f were accurately aligned,the central axis 22 of each annular TSV 14 would align. Clearly, thecentral axes 22 of the TSVs 14 are shifted with respect to each other.This imprecise alignment affects electrical communication between thechips and impacts the reliability of device performance.

BRIEF SUMMARY

The present disclosure is directed to alignment structures that enableprecise alignment of a plurality of wafers or individual die to bealigned and bonded to form a multichip package. The alignment structuresare a plurality of electrically or optically conductive columns thatextend completely through the die or wafer, from a top surface to abottom surface. The columns are formed in selected patterns that canvary in the number of columns in the pattern and location of the columnswith respect to each other.

Precise alignment of the plurality of wafers or individual die isachieved by automated equipment that determines when the patterns ofadjacent wafers are accurately aligned. When the patterns are aligned,the equipment will detect a signal through both wafers through each ofthe columns in the pattern.

For example, a first die and a second die may have a pattern of columnsformed at a respective corner of each die. If the first and second dieare roughly positioned where some of the columns of the first die arealigned with respective columns on the second die, the equipment willdetect some amount of the signal passing through the columns of thefirst and second die. Accordingly, the equipment will make smalladjustments to achieve more precise alignment.

Alternatively, if the first die is incorrectly positioned with respectto the second die, such that no columns of the pattern are overlyingeach other, the equipment will not detect the signal. A larger movementwill be made to align the columns. If some of the columns are nottransmitting the signal, the equipment will sense the columns that arenot aligned and make some movement to achieve more precise alignment.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The foregoing and other features and advantages of the presentdisclosure will be more readily appreciated as the same become betterunderstood from the following detailed description when taken inconjunction with the accompanying drawings.

FIG. 1 is a cross-sectional view of a vertically stacked wafer levelstack using through silicon vias for alignment;

FIG. 2A is an isometric view of a plurality of wafers to be alignedaccording to an embodiment of the present disclosure;

FIG. 2B is a schematic view of the wafers of FIG. 2A after beingaligned;

FIGS. 3A-3C are cross-sectional views of wafers having alignmentstructures according to an embodiment of the present disclosure;

FIG. 3D is a top plan view of a wafer having a plurality of alignmentstructures;

FIG. 4 is an isometric view of a cross-section taken through 4-4 of FIG.3D for two wafers to be bonded;

FIG. 5 is an isometric view of the two wafers of FIG. 4 after alignmentand bonding;

FIGS. 6A and 6B are a top plan view and an isometric view, respectively,of a plurality of die aligned with a plurality of alignment structures;

FIG. 7 is a top plan view of die showing positions of alignmentstructures in the kerf between die;

FIG. 8 is a top plan view of a wafer showing positions of alignmentstructures in the kerf between die according to another embodiment ofthe present disclosure;

FIG. 9 is a top plan view of a wafer having a plurality of alignment diespaced intermittently on the wafer according to yet another embodimentof the present disclosure;

FIGS. 10A-10D are top plan views of various patterns and types oftransmission columns of alignment structures according to the presentdisclosure;

FIG. 11 is a cross-sectional view of capacitive alignment structuresformed on a plurality of wafers in accordance with another embodiment ofthe present disclosure;

FIG. 12 is a cross-sectional view of alignment structures coupled toalignment pads according to an embodiment of the present disclosure;

FIG. 13 is a top plan view of a plurality of alignment pads on the waferin accordance with an embodiment of the present disclosure; and

FIG. 14 is an isometric view of a cross-section taken through 14-14 ofFIG. 13 of one of the alignment pads illustrated in FIG. 13.

DETAILED DESCRIPTION

In the following description, certain specific details are set forth inorder to provide a thorough understanding of various embodiments of thedisclosure. However, one skilled in the art will understand that thedisclosure may be practiced without these specific details. In someinstances, well-known structures associated with the manufacturing ofsemiconductor wafers have not been described in detail to avoidobscuring the descriptions of the embodiments of the present disclosure.

Unless the context requires otherwise, throughout the specification andclaims that follow, the word “comprise” and variations thereof, such as“comprises” and “comprising,” are to be construed in an open, inclusivesense, that is, as “including, but not limited to.”

Reference throughout this specification to “one embodiment” or “anembodiment” means that a particular feature, structure or characteristicdescribed in connection with the embodiment is included in at least oneembodiment. Thus, the appearances of the phrases “in one embodiment” or“in an embodiment” in various places throughout this specification arenot necessarily all referring to the same embodiment. Furthermore, theparticular features, structures, or characteristics may be combined inany suitable manner in one or more embodiments.

In the drawings, identical reference numbers identify similar featuresor elements. The size and relative positions of features in the drawingsare not necessarily drawn to scale.

Vertically stacking wafers or die to form multichip packages offersmanufacturers advantages over traditional single chip packages. Theprocesses to manufacture a single die to include multiple devices aremore complex and time consuming than preparing a single die to have onetype of device. Vertically stacked chips can combine mixed signaltechnologies, such as wireless and optical; MEMS, such as microfluidicsand sensors; and logic. Many of these technologies have enhancedproperties when formed from specific semiconductor materials that havedifferent lattice structures, such as silicon, gallium arsenide, andindium phosphide. Integrating technologies formed with different latticestructures can result in disadvantageous lattice mismatch and problemswith device reliability.

Vertically stacked multichip devices formed from a plurality ofindependently processed wafers avoid problems from lattice mismatch andother process incompatibilities. The individual processing ofhomogeneous technologies on single wafers improves performance,functionality, and increases density.

The accuracy of alignment between the wafers or die to be bonded impactsthe manufacturer's ability to produce reliable vertically stackeddevices. The size and pitch of the alignment structures are factors inaccurately aligning wafers or die.

FIGS. 2A and 2B show a method of aligning a plurality of wafers 102a-102 e to form a vertical stack 100 with alignment circuitry 110. Thewafers 102 a-102 e may have a plurality of alignment structures 106a-106 e in a single alignment die, in a plurality of alignment die, orspaced intermittently at a plurality of locations on the wafers. It mayhave course alignment structures 108 a-108 e and fine alignmentstructures 106 a-106 e. For example, the alignment structures 106 and108 may be formed in a kerf region between die, within each die, orwithin a select number of the die. For ease of illustration, a singlefine alignment die 106 a-106 e is illustrated on each wafer 102 a-102 e.The details and variations on the alignment die and the alignmentstructures will be described in more detail below.

Each wafer includes a plurality of die 104 a-104 e that are manufacturedto have specific device characteristics designed to interact with otherdie vertically stacked in a multichip single package. The first wafer102 a may be a processor or an application specific integrated circuit(ASIC) that is configured to interact with sensors formed on the fourthwafer 102 d. The processor of the first wafer 102 a may be coupled tomemory formed on the second and third wafers 102 b and 102 c to storeinformation received from the sensors. A radio frequency transmitter andreceiver may be formed on the fifth wafer 102 e that is configured tosend information stored in the memory and receive commands from anexternal controller.

Each of the wafers 102 a-102 e are formed independently from each other.Manufacturers may utilize existing semiconductor processing equipmentand techniques to form each of the wafers 102 a-102 e. In oneembodiment, one or more of the wafers 102 a-102 e are made of differentmaterials from each other. By processing incompatible technologiesindependently, the processing time per individual wafer may besignificantly reduced and result in higher yields per wafer. Processingthe wafers individually also reduces complexity and reduces the numberof steps. Individual wafers 102 a-102 e may be processed in differentfabrication locations by different producers. This will enable higherthroughput, enhanced cleanliness, and flexibility in processing for eachwafer. These alignment structures allow standard fabrication equipmentto be used instead of requiring a manufacturer to invest in additionalequipment.

The alignment circuitry 110 in FIG. 2B is coupled to the plurality ofalignment structures 106 e that extend from a first surface 124 of thewafer 102 e and also to the plurality of alignment structures 106 a thatextend from a second surface 126 of the wafer 102 a. In one embodiment,each alignment structure 106 includes a plurality of transmissioncolumns 112 formed in a pattern through each wafer 102. During thedesign process, the manufacturer of the stacked die package determineswhere and in what pattern to form the alignment structures on each waferso that after manufacturing, the wafers or dies can be accuratelyaligned and bonded.

The alignment circuitry 110 may be included in alignment equipmentconfigured to send test signals and adjust the relative positions thewafers 102 a-102 e based on the test signals to achieve precisealignment. A person may also move the wafers relative to each other toachieve proper alignment while looking at the output of the testsignals, or, alternatively, it can be done 100% by computer control,motor drives, and feedback circuits. In one embodiment, the alignmentequipment aligns two wafers 102 a and 102 b by aligning a plurality ofgross alignment structures 108 a and 108 b. Subsequently, the alignmentcircuitry couples a probe to the alignment structures 106 b that extendsfrom the first surface 124 of the wafer 102 b and another probe to thealignment structures 106 a that extends from the second surface 126 ofthe wafer 102 a.

By using the gross alignment structures, the wafers 102 a and 102 b areinitially correctly oriented with respect to each other. After thealignment circuitry 110 makes contact with the alignment structures ofeach wafer, the alignment circuitry 110 sends the signal from the probethrough the plurality of transmission columns 112 of the alignmentstructure 106 b of the wafer 102 b. If none of the correspondingtransmission columns 112 of the wafer 102 a are in contact with thetransmission columns 112 of the wafer 102 b, then no signal will bereceived at the second surface 126 of the wafer 102 a. The alignmentcircuitry then adjusts the position of the wafers with respect to eachother and resends the signal until the signal is received through eachtransmission column of each wafer 102 a.

Once some of the signal is transmitted from the probe through bothwafers 102 a and 102 b, the alignment circuitry can determine which onesof the transmission columns are transmitting the signal through bothwafers. The alignment circuitry is configured to extrapolate adjustmentsto make in order to have each of the transmission columns of the wafer102 b aligned and transmitting the signal through the relatedtransmission columns of the wafer 102 a. The manufacturer may developpatterns that assist the alignment circuitry in determining how toadjust the wafers to achieve alignment.

For electrically conductive transmission columns 112, alignment may beachieved by applying a current to the first surface 124 of the wafer 102b and detecting an amount of that current at the second surface 126 ofthe wafer 102 a. The current detected at the second surface 126 of thewafer 102 a will be the greatest when the first and second ends 118 and120 of the two wafers 102 a and 102 b are aligned. Once the first andsecond wafers 102 a and 102 b are aligned, the alignment process may berepeated by positioning the third wafer 102 c on the vertically stackedwafers, then wafer 102 d, and so forth.

In one embodiment, the transmission columns are electrically conductiveand have a known electrical resistance. The resistance is sufficient tobe some detectible value and may be in the range of 5-10 ohms. Thealignment circuitry 110 transmits the current through the grosslyaligned wafers and detects the resistance through the transmissioncolumn of a number of stacked wafers.

In the simple case of aligning the first and second wafers 102 a and 102b, consider the alignment structure to have three transmission columnsarranged in a pattern. The probe will transmit the signal through allthree ends on the first surface 124 of the wafer 102 b. The second probewill then detect the resistance through the three transmission columns.If two of the three transmission columns are aligned in some way, thesecond probe will detect a resistance. If the third transmission columnof the second wafer 102 b is not aligned with the correspondingtransmission column of the first wafer 102 a, the second probe will notdetect the signal. The alignment circuitry will make slight movements ofthe second wafer with respect to the first wafer to align the thirdtransmission column. The alignment circuitry may be programmed to knowthe pattern of the transmission columns to better extrapolate whatmovements to make to precisely align the wafers. When all three of thetransmission columns are aligned, the resistance will be at a minimum.

In another embodiment, instead of detecting the individual resistance,the probe of the alignment circuitry 110 will transmit the signalthrough all three transmission columns on the second wafer 102 b. Thesecond probe detects an overall resistance through the transmissioncolumns. If one of the transmission columns is not aligned, theresistance will be higher than if all of the transmission columns arealigned. Slight or minor adjustments may be made by the alignmentcircuitry to place the transmission columns in accurate alignment.

In another embodiment, the three transmission columns are formed asparallel resistors. A pad may be formed to electrically communicate withthe ends of each of the three transmission columns. The pad may beformed during the manufacturing process or the pad may be applied as acomponent of the probe. The alignment circuitry will transmit a currentthought the pad and through each of the three transmission columns.Instead of determining the individual resistance through each column,the alignment circuitry evaluates the parallel resistance of all three.

As is known, if resistors are in parallel, the total resistance isreduced. The well-known equation for parallel resistance may be used todetermine how many of the resistors are properly aligned,

$R_{equivalent} = {\frac{1}{R_{1}} + \frac{1}{R_{2}} + {\frac{1}{R_{3}}\mspace{14mu} \ldots \mspace{14mu} {\frac{1}{R_{n}}.}}}$

If the resistors R₁-R_(n) all have equal value, then it can be measuredexactly how many are properly aligned by measuring the total resistanceat large pads on each wafer or die. Alternatively, each resistor mayhave a different value and they may vary from each other by factors of2, 5, or 10. In this way, a measure of total resistance will provideinformation regarding the exact current alignment and also whichresistors are not yet aligned. This will provide information regardinghow much and in what direction to move the wafers relative to each otherto obtain proper alignment.

In an alternative embodiment where the transmission columns areoptically conductive, the alignment circuitry 110 may be configured todetect an intensity of optical light that passes through thetransmission columns of the alignment die 106. The alignment circuitrypositions a light source overlying the ends of the transmission columnson the top wafer, such as wafer 102 b. A light detector is positioned onthe second surface of the last wafer, such as 102 a. Precise alignmentis achieved when an intensity of light detected is at a maximum.Optically conductive transmission columns may be in direct contact withthe respective transmission columns or the ends of related columns maybe spaced by some distance, as described with respect to FIG. 3C,explained later herein.

In yet another embodiment, the alignment structures may be formed asmetal plates adjacent the top and bottom surface of each wafer. As shownin FIG. 11, the wafers are formed having the specific patterns ofcapacitive alignment structures. The alignment circuitry is configuredto adjust the wafers with respect to each other to achieve a maximumcapacitance between capacitive plates of the individual alignmentstructures. The wafers are first initially roughly aligned such that theplates of the capacitive alignment structures are partially overlapping.The alignment circuitry associated with each individual alignmentstructure may be activated at different times to enhance the precisionof alignment.

For example, if the plates of the associated alignment structures arepartially aligned, such that the first plate is shifted to the rightwith respect to the second plate, the alignment circuitry will detect afirst capacitance. The alignment circuitry may shift the first platefurther to the right and detect a second capacitance that will besmaller than the first capacitance. Since the capacitive value is lower,the alignment circuitry may shift the first plate back to the left by anamount equal to the first shift and by an additional amount. A thirdcapacitance will be larger than the first capacitance because more areaof the plates will be aligned.

The alignment process may be automated, such that a robotic arm or otherwafer transportation device places the second wafer 102 b overlying thefirst wafer 102 a in rough alignment. Mechanical adjustment devices orrobotic arms may be incorporated with the alignment circuitry toautomatically make the slight adjustments to align the wafers. Aprocessor and memory may be associated with the alignment circuitry andthe mechanical arms to store information about the type of alignmentstructure and the particular patterns that are associated with a singlewafer or a batch of wafers. A single wafer may include both optical andelectrical transmission columns and capacitive alignment structures. Thememory can store the information specifying where and what type ofalignment structures are on the wafers.

In one embodiment, the manufacturer enters information about the patternof transmission columns associated with a plurality of wafers to bealigned. The information includes the type of transmission columns andthe various patterns formed on the wafers. The processor evaluates theinformation acquired by the alignment circuitry with regard to thepatterns stored in memory to determine the type and amount of adjustmentto be made.

When designing vertically stacked multichip packages, manufacturers mustweigh the benefits of forming wafers of homogenous devices with thepossible inefficient use of real estate on each wafer. For example, theamount of memory desired for a specific multichip device may not coverthe same amount of area as a processor or MEMS to be included in themultichip device. Therefore, the manufacturer must decide whether or notto form the memory to cover the same area as the processor or MEMS or toleave the space unused. More particularly, the kerf regions between thedie of memory will be larger than the kerf regions between die ofprocessors. In some situations, the manufacturer may choose to leave thespace unused and thereby avoid the additional costs associated withadditional processing. Alternatively, the additional test structures orthe alignment circuitry may be formed in the larger kerf regions of thememory.

FIG. 3A shows a cross-sectional view of an embodiment of a verticallystacked multichip device 100 having a plurality of alignment structures106 formed on each wafer 102 a-102 e. The alignment structures 106 areformed during manufacturing of each individual wafer 102 a-102 e atidentical corresponding locations selected by the manufacturer.

Each alignment structure 106 includes a plurality of transmissioncolumns 112 that extend through each wafer from a first surface 124 to asecond surface 126. Precise alignment for each alignment structure 106is achieved when each transmission column 112 of the first wafer 102 ais aligned with a respective one of the transmission columns 112 of thesecond wafer 102 b for a particular alignment structure 106. When allalignment structures 106 are properly aligned, the precise alignment forthe entire wafer is achieved.

After all alignment structures 106 on the wafers are aligned, the wafersare bonded to each other. In one embodiment, the bonding is achieved bya dielectric polymer glue that is applied to thinned wafers to create avoid free bond. The bonding conditions are selected to be compatiblewith CMOS (complementary metal oxide semiconductor) processing. Prior toaligning and bonding, the wafers may be thinned by a chemical mechanicalpolishing or other etch to an etch stop layer, such as an implantedlayer, an epitaxial layer, or a buried oxide layer. Alternatively, twowafers may be aligned and bonded and subsequently thinned. Thinning ofthe wafers results in a final smaller package because an unused portionof the bulk silicon is removed. Methods of bonding thinning are known inthe art and will not be described in detail herein.

In the embodiment of FIG. 3A, each of the wafers 102 a-102 e shows fouralignment structures 106 in this cross-section, each having fourtransmission columns 112 visible. The alignment structures 106 mayinclude any number of transmission columns formed in a pattern. Forexample, the alignment structure 106 may be a 4×4 array of transmissioncolumns 112. Other patterns and arrangements of transmission columnswill be discussed in more detail below.

The transmission columns 112 are electrically or optically conductive.Metal, polysilicon, or other conductive materials may be used to formelectrically conductive transmission columns 112. Glass or othermaterials that allow sufficient light transmission may be used to formoptically conductive transmission columns 112. If an electricalconductor is used for the transmission column 112, then electricalequipment is used to test the resistance of the alignment structuresfrom the first surface 124 of the fifth wafer 102 e to the secondsurface 126 of the first wafer 102 a. If an optical transmission columnis used, then a light source is placed on the first surface 124 of thefifth wafer 102 e and a light detector is placed on the second surface126 of the first wafer 102 a. The wafers are moved with respect to eachother until the maximum light is received by the optical detector andthe wafers are aligned.

In one embodiment, each transmission column 112 extends past the firstand second surface 124, 126 of each wafer by a small distance asillustrated in FIG. 3B. The extension of the transmission columns 112assists in permitting electrical or optical communication to passthrough aligned transmission columns 112 from wafer 102 e to wafer 102d. The extension can be very small, for example, in the range of a fewmicrons, to ensure the transmission columns 112 contact each otherinstead of the wafer surfaces contacting.

FIG. 3C is an alternative embodiment showing the fourth wafer 102 dspaced from the fifth wafer 102 e by a distance 128. The transmissioncolumns 112 of the two wafers are not contacting each other. Thisconfiguration may be used for the optical transmission described above.Precise alignment is achieved when the maximum light is transmittedthrough the transmission columns 112 from the fifth wafer 102 e throughthe fourth wafer 102 d.

In one embodiment, the distance 128 between the wafers is related to adistance of electrical contact pads or balls in a ball grid array thatenable operational electrical communication between devices. Thealignment is achieved through precise x-y alignment. The vertical zdistance may vary between the wafers depending on the surfacecharacteristics of each wafer.

The alignment structures 106 may be formed in a kerf or scribe regionbetween a plurality of die formed on the wafer. The kerf or scriberegion may be 100 microns in size, which provides space to form avariety of the alignment structures in different patterns. The alignmentstructures 106 may be formed within each die in order to allow for dielevel alignment. In another embodiment, specific die regions may bedesignated for alignment and will only contain alignment structures. Thevarious embodiments of the alignment structures and their positioning onthe wafers will be discussed in more detail below.

FIG. 3D is a top view of the wafer 102 a having a plurality of die 104 amanufactured for vertically stacked integration that include a pluralityof alignment structures 106 formed in a kerf 130 between the die 104 a.In this embodiment, the alignment structures 106 each include sixtransmission columns 112 arranged in a 2×3 array. The alignmentstructures 106 are positioned in a variety of locations to avoid a falsepositive determination of alignment. If one of the wafers is slightlyshifted, the intermittently formed alignment structures 106 preventincorrect alignment with an adjacent alignment structure.

The die 104 a each have a first end 132 and a second end 134 that aretransverse to a first side 136 and a second side 138. Some of thealignment structures 106 are positioned in the kerf 130 between thefirst end 132 of one die 104 a and a second end 134 of an adjacent die.Other alignment structures 106 are positioned between the first side 136of one die and the second side 138 of an adjacent die. Additionalalignment structures 106 are positioned in a center of an intersectionof kerf regions 130 adjacent corners of four die.

FIGS. 4 and 5 are isometric views of a cross-section taken through thefirst wafer 102 a and the second wafer 102 b having alignment structurespositioned as illustrated in FIG. 3D. The second wafer 102 b includesthe plurality of die 104 b separated by the kerf or scribe line 130.Each die includes the first end 132, the second end 134, the first side136, and the second side 138. The plurality of alignment structures 106are positioned on the second wafer 102 b in the same arrangement asdescribed above in FIG. 3D.

The cross-section is taken through two alignment structures 140 and 142,which are both 2×3 arrays of transmission columns 112. The transmissioncolumns 112 extend from the first surface 124 to the second surface 126of each wafer. In this embodiment, both the second wafer 102 b and thefirst wafer 102 a are thinned prior to alignment. In an alternativeembodiment, the first wafer 102 a may be thinned after alignment andbonding with the second wafer 102 b.

The first wafer 102 a includes alignment structures 140 a and 142 apositioned in a location that corresponds to the alignment structures140 b and 142 b of the wafer 102 b. The die 104 a and the alignmentstructures of the first wafer 102 a are the same size and shape as thedie 104 b and alignment structures of the second wafer 102 b. Shown inFIG. 5, the alignment structures 140 b and 142 b are perfectly alignedwith the alignment structures 140 a and 142 a of the first wafer 102 a.Once alignment is achieved, the wafers are bonded, diced, and packagedto form multichip single packages.

The die 104 b of the second wafer 102 b and the die 104 a of the firstwafer 102 a may form different devices that are traditionally difficultto integrate into a single package. The manufacturer designs the devicesto be in electrical communication once the second wafer 102 b and thefirst wafer 102 a are aligned.

In an alternative embodiment, the alignment circuitry is configured toindividually probe each column. Two transmission columns A1, A2 of thealignment structure 140 a on the first wafer 102 a correspond to twotransmission columns B1, B2 of the alignment structure 140 b on thesecond wafer 102 b. Once the transmission columns A1 and B1 are aligned,the alignment circuitry can probe the A1, B1 pair and detect aresistance. Subsequently, the alignment circuitry can probe the A1, B2pair. If the wafers are not correctly aligned, the alignment circuitrymight detect some amount of current through the A1, B2 pair. Slightadjustments may be automatically made by the alignment circuitry untilno current passes through the A1, B2 pair. The testing of an alignedpair of transmission columns and the adjacent transmission columnprovides for more accurate alignment of the wafers.

FIG. 6A is a top down view of a wafer 202 showing another alternativeembodiment of alignment structures 206 a and 206 b formed within eachdie 204 a and 204 b instead of in the kerf 130. Forming the alignmentstructures 206 in each die 204 provides manufacturers with flexibilityfor when to perform the alignment, wafer level or die level. In thisembodiment, adjacent die include different numbers of transmissioncolumns 112 in different patterns.

The alignment structures 206 a are formed in the 2×3 array discussedabove. However, the alignment structures 206 b are formed in a 2×6array. The different patterns formed on adjacent die 204 a and 204 bprevent incorrectly aligning adjacent die. Other patterns may be usedfor a plurality of adjacent die to ensure that a shift of one die withrespect to another die does not indicate alignment has been achieved.

The transmission columns 112 of the three alignment structures 206 aformed on the die 204 a are located in a first corner 218, a secondcorner 220, and a third corner 222. The transmission columns 112 in thefirst corner 218 and the third corner 222 are positioned so three of thetransmission columns are parallel to the first end 132 and the secondend 134, respectively. In contrast, the transmission columns 112 in thesecond corner 220 are positioned so three of the transmission columns112 are parallel to the first side 136. The difference in orientation ofthe alignment structures is also presented on the second die 204 b.

Including at least three non-symmetrical alignment structures 206 a and206 b in each of the die 204 a and 204 b is another precaution that canassist in preventing false alignment. For example, if a first die and asecond die are to be aligned, but the first die is rotated 180 degreeswith respect to the second die, the alignment structures 206 a thatshould align in the second corner 220 will not be in communication. Moreparticularly, the second corner 220 of the first die will overlie acorner of the second die that does not include any transmission columns.

FIG. 6B is a complete stack 200 of five die 204 a-204 e formed inaccordance with the configuration of the die 204 a of FIG. 6A. In thisembodiment, the alignment structures 206 a form the 2×3 array oftransmission columns 112 in the first orientation in the first and thirdcorners 218 and 222. The other 2×3 array of transmission columns 112 isin a second orientation that is rotated 90 degrees with respect to thefirst orientation in the second corner 220.

FIG. 7 is an alternative embodiment of the alignment structures 206positioned in the kerf 130 between two die 204. The transmission columns112 are more tightly packed than in FIG. 6A. The tighter arrangementallows two 2×3 arrays to be formed side by side in the kerf 130 betweenthe two die 204. The pattern of two 2×3 arrays formed between the firstside 136 of one die and the second side 138 of the other die may berepeated at various locations on the wafer to avoid incorrect alignment.

FIG. 8 illustrates an alternative embodiment of the wafer 202 having theplurality of die 204 separated by the kerf 130. The plurality ofalignment structures 206 are formed in the kerf 130 and are configuredto align a plurality of wafers to make a vertically stacked wafer levelstack. In this embodiment, the alignment structures 206 include sixtransmission columns 112 formed in a 2×3 array.

In this embodiment, a pattern 226 includes three alignment structures206 positioned in the intersection of transverse kerf regions 130. Thepattern 226 is repeated in each intersection of kerf regions indifferent orientations, which are shifted from each other by 90 or 180degrees. This is another alternative arrangement that assists inpreventing false alignment.

FIG. 9 is an alternative embodiment of the wafer 102 having a pluralityof die 104 formed thereon. Instead of having a plurality of alignmentstructures formed in a kerf region, a plurality of die regions aredesignated to be alignment die 106 a-106 e. Each of the alignment die106 includes a plurality of transmission columns 112 formed in apattern. In this embodiment, there are five different alignment die 106intermittently positioned across the wafer, each having a differentpattern of transmission columns 112.

A first pattern 144 a of the transmission columns 112 is formed throughthe alignment die 106 a. This alignment die 106 a is formed at anexterior edge of the die region of the wafer 102. A second pattern 144 bof the transmission columns 112 is formed in the alignment die 106 b.This alignment die 106 b is formed in a second row of die on the wafer102. The first pattern 144 a and the second pattern 144 b have adifferent number of transmission columns that are arranged differently,which ensures there is no chance of incorrect alignment of this waferwith an additional wafer. Accordingly, the additional alignment die 106c, 106 d, 106 e also have different patterns 144 c, 144 d, 144 e,respectively. Each of the alignment die are formed in locations of thewafer 102 that are distinct from each of the other alignment die 106 sothat if one wafer is shifted 90 or 180 degrees with respect to anotherwafer, incorrect alignment will not occur. In addition, if one of thewafers to be stacked is shifted by some amount, the alignment die 106will not falsely align.

As described above, gross alignment of a plurality of wafers 102 may beachieved by an automated system that aligns gross alignment structures108. The gross alignment structures 108 may be large holes formedthrough each wafer is non-symmetrical positions. Three gross alignmentstructures 108 are formed on the wafer 102, however, any number of grossalignment structures 108 may be formed.

FIGS. 10A-10D illustrate alignment structures formed having a pluralityof transmission columns 112 in a variety of different patterns. Inaddition, the transmission columns 112 may be formed in different sizesand shapes, with different materials, and spaced in different ways. Forexample, FIG. 10A illustrates an alignment structure 106 that includes a4×4 array of transmission columns 112. The transmission columns 112 areevenly spaced from each other to make a square shape.

FIG. 10B illustrates another alignment structure 106 having a pluralityof transmission columns 112 formed to be annular rings. The rings may bemetal or other conductive material. The transmission columns 112 arealso spaced evenly and form a 4×4 square pattern. This alignmentstructure 106 may be formed in the kerf region or within a plurality ofdie. Alternatively, as described above, the alignment structures 106 maybe formed in a die region designated for alignment purposes. Designatingseveral die regions for alignment purposes prevents the kerf region frombeing cluttered with alignment structures and provides space for teststructures to be formed.

FIG. 10C illustrates an alternative pattern for an alignment structure106 that includes a plurality of transmission columns 112. In thisembodiment, the plurality of transmission columns 112 are formed in anirregular pattern and spaced by a small distance. The irregular patternof the alignment structure 106 decreases the chance of false alignment,where the transmission columns are all transmitting the signal throughboth wafers, but the wafers are not actually correctly aligned.

FIG. 10D illustrates an alternative embodiment of transmission columns782 in an alignment structure 106 where the transmission columns 782 areformed from different conductive materials having different resistancevalues. For example, one column 112 may have a resistance of 5 ohms,where an adjacent column has a resistance of 50 ohms. Other columns inthe alignment structure 106 may have a resistance of 5 k ohms. In oneembodiment, the transmission columns have resistance values that differfrom each other by factors of 5 or 10.

The alignment structure 106 is a 3×3 array of the transmission columns112 that also form a square pattern. The different types of metals willresult in different resistances detected through the transmissioncolumns when they are aligned. Therefore, the alignment circuitry canuse the variations in resistance as another factor to evaluate todetermine the adjustments to make. The variations of materials andpatterns are provided for illustrative purposes and other variations maybe utilized in accordance with the present disclosure.

FIG. 11 is a cross-section of a vertically stacked structure 300 havinga plurality of capacitive alignment structures 306 according to analternative embodiment of the present disclosure. The capacitivealignment structures 306 are formed on four wafers 302 a-302 d that havebeen aligned and bonded to form the vertically stacked structure 300.The alignment may be achieved at the wafer level or at the die level.

In this embodiment, each wafer may have a different pattern of alignmentstructures 306 on a first and second surface. For example, the wafer 302b has a first pattern 314 b adjacent a first surface 324 and a secondpattern 316 b adjacent a second surface 326. The first wafer 302 aadjacent the second wafer 302 b has the first pattern 314 a of alignmentstructures 306 that are in the same formation as the first pattern 314b. In addition, the wafer 302 c adjacent the second wafer 302 b has thesecond pattern 316 c of alignment structures 306 that are in the sameformation as the second pattern 316 b.

Each alignment structure 306 includes a first and a second capacitiveplate formed on a first and a second wafer, respectively. Moreparticularly, the wafer 302 a includes three capacitive plates 318 a,320 a, and 322 a formed adjacent the first surface 324 of the wafer 302a that is facing the second surface 326 of the wafer 302 b. Each of thecapacitive plates 318 a, 320 a, and 322 a forms one plate of each of thecapacitive alignment structures 306 of the first pattern 314 a, 314 b.Three capacitive plates 318 b, 320 b, and 322 b formed adjacent thesecond surface 326 of the wafer 302 b correspond with the threecapacitive plates 318 a, 320 a, and 322 a of the wafer 302 a to form thealignment structures 306 of the first pattern 314.

The second pattern 316 b and 316 c of alignment structures 306 is formedbetween the wafers 302 b and 302 c. Incorrect alignment or falsealignment is avoided by including different patterns of alignmentstructures on opposing sides of each wafer. The capacitive alignmentstructures do not extend from the first or second surface of each wafer.The wafers can be thinned prior to alignment and bonding withoutdamaging the alignment structures or affecting the accuracy ofalignment.

Once the wafers are formed having the specific patterns of capacitivealignment structures, alignment is achieved by maximizing thecapacitance between capacitive plates of the individual alignmentstructures. For example, if the wafer 302 b is shifted with respect tothe wafer 302 a, a capacitance detected between the capacitive plates318 a and 318 b of the wafers 302 a and 302 b will be lower than if thecapacitive plates are aligned.

As with the transmission columns, a similar complexity of patterns canbe achieved with the capacitive plates. In addition, the wafers havingcapacitive alignment structures may be positioned to have a distancebetween the wafers as shown in FIG. 3C. The distance may relate to asize of the bonding or contact pads that are used to electricallyconnect the multiple die for operation. The wafers may be separated byair or by a dielectric bonding substance. The capacitive alignmentstructures have the further advantage that circuits can be formed on thesame wafer or die that interface with the capacitors and provide anindication of alignment. Circuits on the same die that sense capacitivevalue can easily be formed using known techniques. When the capacitancereaches a desired value, the on chip circuit can output a matchingsignal indicating that the wafers or die are now properly aligned.

FIG. 12 is a cross-sectional view of an alternative embodiment of thevertically stacked structure 100 having the plurality of alignmentstructures 106 configured to align the plurality of wafers 102 a-102 e.Each alignment structure 106 includes a plurality of transmissioncolumns 112 formed through each wafer, in electrical communication withan alignment pad 146 that is formed on the second surface 126 of eachwafer 102. The pad 146 is in contact with each transmission column 112of one alignment structure 106.

FIGS. 13 and 14 illustrate an alternative embodiment showing theplurality of alignment structures 106 having alignment pads 146 formedon the wafer 102. FIG. 13 is a top down view of the plurality ofalignment structures 106 formed at various locations between theplurality of die 104 on the wafer 102. The alignment structures 106include the plurality of transmission columns 112 formed through thewafer 102 that are in electrical communication with the alignment pad146.

The alignment pad 146 is sized and shaped to correspond to a pattern ofthe transmission columns 112. The alignment pads 146 and thecorresponding transmission columns 112 are formed intermittently acrossa wafer to avoid incorrect alignment. The alignment pads 146 are a thinelectrically or optically conductive material.

In one embodiment, the transmission columns 112 are formed through eachwafer from the top surface 124 to the bottom surface 126. If the wafersare thinned, the alignment pads 146 may be formed after the thinningprocess in order to avoid inadvertent damage to the pads 146. The padsalso provide a surface for the transmission columns 112 to interact withto ensure electrical or optical communication through two or morewafers. In addition, if one of the transmission columns is damagedduring thinning or improperly formed, the alignment pad will still makecontact with other transmission columns and provide the transmissionneeded for alignment.

FIG. 14 is a cross-section of FIG. 13 through 14-14. The alignmentstructure 106 includes the plurality of transmission columns 112 formedthrough the wafer 102 and the alignment pad 146 in communication withthe transmission columns. In this embodiment, the alignment structuresare formed in the intersection of kerf regions 130. As described above,the alignment structures may be formed in the kerf region or in the dieat a variety of locations selected by the manufacturer. The alignmentpads 146 may be also be used as the electrical connection for operationas well as for alignment.

In addition to providing for alignment of wafers, the alignmentstructures 106 described herein aid in heat dispersion in the chippackages. The transmission columns that remain after dicing the wafersmay act as a heat sink that dissipates heat away from the active areas.

The use of these densely arranged transmission columns also improveplanarization results when ultra-low k (ULK) dielectrics are used.Ultra-low k dielectrics are used to overcome device speed limitations.In some embodiments, the ultra-low k replaces silicon dioxide in regionsof the wafers. Ultra-low k can have a 50% lower dielectric constant thansilicon dioxide. The ultra-low k can reduce parasitic capacitance,increase switching speeds, and lower heat dissipation by improvingdensity and introducing porosity into the wafer.

For example, the ultra-low k can erode in areas between transmissioncolumns during a chemical mechanical polish. One embodiment thatimproves ultra-low k loss during chemical mechanical processing, isforming the columns to have a width separated from each other by adistance, where the width and the distance are equal. More particularly,a one to one ratio of column width to distance between the columnsresults in the least amount of ultra-low k loss during planarization.The improved planarization also improves wafer to wafer adhesion becauseof the improved surface quality.

The various embodiments described above can be combined to providefurther embodiments. Aspects of the embodiments can be modified, ifnecessary, to employ concepts of the various patents, applications andpublications to provide yet further embodiments.

These and other changes can be made to the embodiments in light of theabove-detailed description. In general, in the following claims, theterms used should not be construed to limit the claims to the specificembodiments disclosed in the specification and the claims, but should beconstrued to include all possible embodiments along with the full scopeof equivalents to which such claims are entitled. Accordingly, theclaims are not limited by the disclosure.

We claim:
 1. A method of aligning a plurality of die, comprising:positioning a first die having a first plurality of alignment structuresoverlying a second die having a second plurality of alignmentstructures, the first plurality of alignment structures having a firstplurality of transmission columns arranged in a pattern and the secondplurality of alignment structures having a second plurality oftransmission columns arranged in the pattern of the first transmissioncolumns; applying a first signal to the first plurality of alignmentstructures; detecting a second signal at the second plurality ofalignment structures; determining a difference between the first andsecond signals; and adjusting a position of the first die with respectto the second die to decrease the difference between the first andsecond signals.
 2. The method of claim 1 wherein applying the firstsignal and detecting the second signal include applying the first signalto a first end of each of the first transmission columns and detectingthe second signal at a second end of each of the second transmissioncolumns, a second end of each of the first transmission columns inelectrical communication with a first end of each of the secondtransmission columns.
 3. The method of claim 2 wherein determining thedifference between the first and second signals includes determining aresistance between the first end of each of the first transmissioncolumns and the second end of each of the second transmission columns.4. A method, comprising: forming a plurality of first transmissioncolumns in a first wafer, the first transmission columns arranged in apattern; forming a plurality of second transmission columns in a secondwafer, the second transmission columns arranged in the pattern; aligningthe first wafer with the second wafer by matching the pattern of thefirst wafer with the pattern of the second wafer, the aligningincluding: applying a signal to the first transmission columns;detecting the signal at the second transmission columns; and adjusting aposition of the first wafer with respect to the second wafer to moreprecisely align the pattern.
 5. The method of claim 4 wherein thealigning includes determining a difference a first value of the signalapplied to the first transmission columns and a second value of thesignal detected at the second transmission columns.
 6. The method ofclaim 5 wherein adjusting the position includes decreasing thedifference between the first and second values.
 7. The method of claim 4wherein applying the signal includes applying an optical signal to anend of at least one of the first transmission columns.
 8. The method ofclaim 7 wherein detecting the signal includes detecting the opticalsignal passing through the at least one of the first plurality ofalignment structures at an end of one of the second plurality ofalignment structures.
 9. The method of claim 4 wherein forming theplurality of first transmission columns includes forming a first end ofthe first transmission columns extending from a first surface of thefirst wafer by a first distance.
 10. The method of claim 9 whereinforming the plurality of first transmission columns includes forming asecond end of the first transmission columns extending from a secondsurface of the first wafer by a second distance.
 11. The method of claim9 wherein forming the plurality of first transmission columns includesforming the first ends and second ends of the first transmission columnsnot overlaying of any portion of the first and second surface of thefirst wafer.
 12. The method of claim 9 wherein forming the plurality ofsecond transmission columns includes forming a first end of the secondtransmission columns extending from a first surface of the second waferby a second distance.
 13. The method of claim 12, further comprisingarranging the first wafer and the second wafer to have the first end ofthe first transmission columns facing the first end of the secondtransmission columns.
 14. The method of claim 12 wherein forming theplurality of second transmission columns includes forming a second endof the second transmission columns extending from a second surface ofthe second wafer by a third distance.
 15. The method of claim 4 whereinforming the plurality of first transmission columns includes forming afirst one of the first transmission columns from a first material andforming a second one of the first transmission columns from a secondmaterial that is different from the first material.
 16. A device,comprising: a first die having a first surface and a second surface; aplurality of first capacitive alignment structures arranged in a firstpattern and formed on the first die closer to the first surface of thefirst die than to the second surface; a second die having a firstsurface and a second surface, the second surface positioned overlyingthe first surface of the first die; and a plurality of second capacitivealignment structures arranged in the same first pattern as the firstcapacitive alignment structures and formed on the second die closer tothe second surface than to the first surface, the second surfaceoverlying the first surface of the first die, the first and secondcapacitive alignment structures configured to form first and secondplates, respectively, of a plurality of capacitors, the plurality ofcapacitors arranged in the first pattern.
 17. The device of claim 16,further comprising: a third die having a first surface and a secondsurface; a plurality of third capacitive alignment structures arrangedin a second pattern and formed in the third die closer to the firstsurface of the third die than to the second surface; the second dieincludes a plurality of fourth capacitive alignment structures arrangedin the second pattern, the fourth capacitive alignment structures areformed in the second die closer to the first surface of the second diethan to the second surface and are configured to form a plurality ofcapacitors with the plurality of third capacitive alignment structuresin the third die.
 18. The device of claim 17 wherein the first patternof the second alignment structures is different from the second patternof third alignment structures on the second die.
 19. A method,comprising: positioning a plurality of first capacitive alignmentstructures on a first die in close proximity to a plurality of secondcapacitive alignment structures on a second die; detecting a firstcapacitance between at least one of the plurality of first capacitivealignment structures and at least one of the plurality of secondcapacitive alignment structures; adjusting a position of the first diewith respect to the second die; detecting a second capacitance betweenthe at least one of the plurality of first capacitive alignmentstructures and the at least one of the plurality of second capacitivealignment structures; comparing the first capacitance to the secondcapacitance; and precisely aligning the first die with the second die byadjusting the position of the first die with respect to the second diebased on the comparing.
 20. The method of claim 19 wherein the firstcapacitive alignment structures are positioned closer to a first surfaceof the first die than a second surface of the first die.
 21. The methodof claim 20 wherein the second capacitive alignment structures arepositioned closer to a first surface of the second die than a secondsurface of the second die.